1. Field of the Invention
The present invention relates to certain improvements in digital video recording.
2. Background Art
Compressed video frames are variable in size depending on the complexity of the picture. Therefore, the recorder must maintain a directory of video frame disk addresses to support cueing (jumping to an arbitrary frame), random access playback (switching seamlessly without hesitation or interruption among material recorded at different physical locations on disk), and “feature” playback (fast forward and reverse scan).
Typically, this frame directory is kept on disk. With this approach, when a user submits a cue command, the recorder must read the directory first to determine the disk address, then read the frame from disk. This latency results in a visible hesitation or requires that the cue command be received some period in advance of the required cue. There is a need to overcome such latency.
Conceptually, recording and playing motion video to and from hard disk is simply performing the conversion from its analog (or uncompressed digital) format to a continuous compressed data stream and managing this continuous stream of data as a series of “writes” or “reads to” or “reads from” hard disk.
The hard disk is the slowest component in the system. To achieve the performance threshold that broadcast quality playback requires, it is necessary to interface to the hard disk at maximum efficiency. The data transfer method must be tailored to the hard disk's characteristics. With modern hard disks, the highest performance interface is achieved with burst data transfers of a size that matches the disk's cache memory. There is a need to implement burst data transfers appropriate for video data at maximum efficiency.
The user configures a recorder to record video at a specified compression ratio, for example, 4:1. The disk may not be able to sustain this specified data rate when the user has installed a low performance disk or if the image data frequency changes from simple pictures (black frames, for example) to complex ones. For example, one may watch MTV for a few minutes for excellent examples of video that stress JPEG compression rate-averaging algorithms. If the disk falls far enough behind, the result is catastrophic: The record stream skips frames, losing them irretrievably. There is a need to control compression so that such catastrophic results are averted.
Certain applications benefit from the ability to record continuously. In surveillance, for example, it is useful to record until an alarm occurs, then to stop some time after the alarm. The recorder has captured time before, during, and after the alarm, which can be useful in determining the cause of the alarm. There is a need to provide support for such a loop recording mode.